Copper nanoparticles prepared by pulsed exploding wire

In this work copper nanopowder was created at different liquidmedias like DDDW, ethylene glycol and Polyvinylpyrrolidone(PVP). Copper nanopowder prepared using explosion wire processand investigated the effects of the exploding energy, wire diameter,the type of liquid on the particle size, and the particles sizedistribution. The nanoparticles are characterized by x-ray diffraction,UV-visible absorption spectroscopy and transmission electronmicroscopy (TEM). The x-ray diffraction results reveal that thenanoparticles continue to routine lattice periodicity at reducedparticle size. The UV-Visible absorption spectrum of liquid solutionfor copper nanoparticles shows sharp and single surface Plasmonresonance (SPR) peak centered at a wavelength of 590 nm inethylene glycol media, but don’t have peak in PVP fluid. This peakindicated the production of pure and spherical copper nanoparticle.


Introduction
Nanoscience and nanotechnology is a promising and fast developing field that broadly encompasses the fundamental understanding and resulting technological advances arising from the utilization of materials having the nanometer length scale [1]. Nanofluids, dispersed nanoparticles suspended in a base fluid, have gained wonderful interest from scientific and industrial communities because of their exclusive properties. They have been used in many industrial applications such as heat transfer, electronic, biomedical device manufacturing, and others [2]. Copper nanoparticles have been prepared using the methods such as thermal reduction, vacuum vapor deposition, microwave irradiation methods, chemical reduction, and laser ablation [3]. In this research the electrical explosion wire (EEW) was adopted because it is a very simple, effectiveness, and low-cost technique to fabricate different types of nanopowders. One can use this method for synthesis nanoparticle in liquid or air. Properties of nanoparticles obtained by EEW depend on several conditions include wire diameter, wire material, capacitor bank voltage, current pulse and properties of the ambient medium. In general, all of these properties could effect on the particles size and size distribution. The media surroundings of the explosion process effects on the size, shape size distribution and chemical reactions between wire materials and active medium. Ambient environment of EEW process in liquid showed a significant effect on the properties of synthesized materials [4,5].
The electrical exploding wire experiment is performed by rapidly discharging an electrically stored source through a fine wire. The capacitor was charged using an applied voltage. The amount of stored energy can be evaluated by: (1) where C and V are the capacitance and charging voltage respectively. In this research, the method is the mixed between exploding wire technique and spark explosion method. When the first electrode (wire) touched the second electrode (plate) the wire vaporized and turned into plasma [5]. After that, wire evaporation, and convert to plasma. The plasma expands in the medium due to the enormous difference in the temperature and pressure between the plasma and the ambient liquid. The expanded plasma particles will gradually cool because of collisions with ambient liquid molecules. Finally, the plasma loses its expansive driving force. Hence, a low ambient pressure allows a large expansion volume. This leads to low number concentration of the substance during particle growth, resulting in small nano-particles. The vaporized particles condensed more efficiently in the liquid than in ambient air [6,7,8].

Experimental work
The schematic diagram of EEW apparatus for the preparation of metal nanoparticles in liquid media was shown in Fig.1. These apparatus consists of chamber for nanoparticles preparation and an electrical discharge circuit for generation of large pulse current. The electrical circuit consists of a capacitors bank 2500µF, 500V dc power supply, wire guides, wire supply unit, represents the first electrode, and the other electrode is a metal plate. When the wire is brought close to the plate as soon as contact is made the storage energy in 2500µF capacitors released through the two electrodes (wire and plate). The wire and the plate at the contact point were over heated, evaporated and converted to plasma state. Copper wires with three exploded in liquid to produce Cu nanoparticles. Three type of liquid were used as a medium represented by double distilled deionized water (DDDW), ethylene glycol, and Polyvinylpyrrolidone (PVP) of 5mM concentration.
The typical experimental conditions are summarized in Table 1: TEM and X-Ray where employed to confirm the nanoparticles shape, size and size distributions. The absorption spectrum of copper colloid was investigated by UV-visible from a double beam spectrophotometer in the wavelength range of 300-800 nm. TEM was used to determine the particle size and particles size distribution. To examine a stability of the prepared nanoparticles, the changes in the absorption spectrum with time were studied.

Results and discussion
The synthesis of the copper nanoparticles was immediately followed by UV-Visible analysis. UVvisible spectrum recorded in the absorbance mode in the range 300-800nm as shown in Fig.2 and Fig. 3 where Fig. 2 for Cu nanoparticles prepared in DDDW and Fig. 3 for cu nanoparticles prepared in ethylene glycol. These figures show a single SPR peak for each curve, which indicates formation of copper nanoparticles. The SPR of NPs is mostly controlled by certain parameters, such as shape, morphology, size, composition and environment dielectric constant of the prepared nanoparticles. UV-Visible absorption spectrum of copper nanoparticles which were prepared in DDDW and ethylene glycol media showed surface Plasmon resonance (SPR) absorption peak centered around (600 nm) for all samples. The central peak position depends on the wire diameter, charging voltage and media type. The SPR has a maximum of absorption peak λ max for all diameters of wire, charging voltage, discharge energy and the types of fluid are shown in Table 2. It is clear that, the optical properties depend strongly on the liquid media. For the same wire diameter and the same charging voltage, the particles were prepared in DDDW were smaller than those prepared in ethylene glycol. The SPR absorption peak for copper nanoparticles prepared in DDDW is sharper than that of nanoparticles prepared in Ethylene Glycol. This means, that the nanoparticles prepared in DDDW have narrow particles size distribution and it has almost the same size.
The particle diameters estimated by SPR absorption peak position shows that the diameter of the particles decreases with decreasing of the metal wire diameter at fixed voltage. While the wire diameter increases, tend to shift the value of the λ max to long wavelength as shown in Table 2 that  means  the  size  of  particles  became larger. The maximum wavelength (λ max ) was shifted to the short wavelength, when the charging voltage increases for the same wire diameter. This means that the size of the particles became less.  Fig.4 shows the UV-visible optical absorption spectra in the range 300-800 nm of the Cu nanoparticles which prepared in PVP (its concentration 5%). PVP media was found to be a very efficient stabilizer for the stabilization of copper nanoparticles. UV-visible optical absorption spectra of the colloid don't have SPR copper peak because of high concentration of PVP. The copper nanoparticles prepared in PVP have large size compared with that prepared in ethylene glycol or DDDW, and these nanoparticles shows weak SPR peak, this means that the nanoparticles coated by PVP. In order to observe a stability of the prepared copper colloid by the changes in the absorption spectrum with time were studied for copper colloid synthesized by EEW using wire with diameter 0.3mm and with charging voltage 250 Volt. The changes in the absorption spectrum with time were shown in Fig.5. The spectral change indicates that the diameters of the nanoparticles are increased more under aging effect, which is interpreted to the metal nanoparticles are aggregated. The changed observed as red shift in SPR peaks position with time. The peak position was shifted to long wavelength and still has the same intensity for fresh nanoparticles, this means that the nanoparticles become larger, but the peak width fixed with time. This confirms that the particles size distribution remains steady. Fig.6 shows two images one for the fresh sample and the other after two months.

Conclusions
The NPs diameters estimated from SPR absorption peak position shows that the diameters of the particles decreases with the decrease of the copper wire diameter, at fixed voltage. For the same wire diameter and the same charging voltage, the particles prepared in DDDW smaller than that prepared in PVP and Ethylene Glycol. The SPR absorption peak for copper nanoparticles prepared in DDDW it has a full width smaller than that for nanoparticles prepared in Ethylene and it have the smallest particles size and narrow particles size distribution. Also for the NPs prepared by the same condition the SPR peak position shifted to the longer wavelength with increase the wire diameter. The copper nanoparticles were prepared in PVP it appeared least effected by preparation condition within the experimental condition used in this work, and the copper nanoparticles capped by thick layer of PVP, this layer prevented the particles from more growth and more agglomeration.